FIG. 1 is a diagram illustrating an example of transmit power control in a mobile communication system. In the mobile communication system, a mobile station 1 controls transmit power to be used to transmit data through an uplink (UL) channel (hereinafter, referred to as “UL transmit power”). A base station 2 receives the data transmitted from the mobile station 1 and detects a reception level thereof via a reception level detecting unit 2a. Then, when the base station 2 determines that the detected reception level is low, the base station 2 sends a command to the mobile station 1 to raise the UL transmit power, via a downlink control channel by a transmit power control (TPC) inserting unit 2b. On the other hand, when the base station 2 determines that the detected reception level is high, the base station 2 sends a command to the mobile station 1 to lower the UL transmit power, via the downlink control channel by the TPC inserting unit 2b. A UL transmit power control command includes a TPC value.
The mobile station 1 receives a command sent from the base station 2 for requesting control of the UL transmit power. Then, a TPC extracting unit 1a of the mobile station 1 extracts a TPC value from the received command requesting control of the UL transmit power. Thereafter, a transmit power control unit 1b controls a UL transmit power value in accordance with the extracted TPC value.
FIG. 2 is a diagram illustrating an example of a transmit power control unit. A TPC value conversion unit 3 converts a TPC value supplied from the TPC extracting unit 1a into a decibel value and supplies the decibel value to a cumulative addition unit 4. The cumulative addition unit 4 then adds an output value from the TPC value conversion unit 3 to a transmit power value of a previous output to obtain an increased or decreased transmit power value. The increased or decreased transmit power value outputted from the cumulative addition unit 4 is added to an initial power value in an addition unit 5, and the result is then outputted as the transmit power value. Note that an up or down range of the transmit power set through a TPC value may be an arbitrary fixed value.
In a mobile communication system, data is handled per frame, which is a time unit. When data is to be exchanged, the data is transmitted or received using sub-frames into which data for a single frame is divided. Thus, a mobile station controls the UL transmit power value on a sub-frame-by-sub-frame basis.
In UL transmit power control in mobile communication systems, a mobile station, upon receiving a UL transmit power control command from a base station, controls the transmit power in accordance with the specified TPC value. However, when the condition of the transmission path from the base station is bad, there are cases where the mobile station mistakenly receives a UL transmit power control command sent from the base station or the base station sends out a wrong command.
In such a case, the mobile station may receive a UL transmit power control command requesting raising of the transmit power even though the mobile station is in a state where the transmit power is supposed to be lowered. In this case, such transmit power control leads to excess transmit power, which in turn may lead to an increase in consumed power at the mobile station, interference with a signal from another mobile station at the base station, and so on.
Here, a technique has been disclosed in which, when a mobile station receives commands to raise the transmit power by an excessive amount, the mobile station carries out mask processing for ignoring UL transmit power control commands from the base station (see, for example, Japanese Laid-open Patent Publication No. 2006-186757). In addition, a technique in which a time alignment (TA) value is used as information indicating the distance between a mobile station and a base station has been disclosed (see, for example, Japanese Laid-open Patent Publication No. 09-284215). Furthermore, a technique has been disclosed in which a table is created by obtaining the maximum transmit power for a distance from each mobile station and the distance from a mobile station is measured on the basis of a difference between a reception timing signal and a reference timing (see, for example, Japanese Laid-open Patent Publication No. 2001-217774).